The present invention relates generally to a rotary grinder with an improved control system and more specifically to a control system capable of minimizing rotor over-current events while maximizing grinding efficiency for a rotary grinder having a rotor and a ram for feeding material into the rotor. Rotary grinders are used to grind a wide variety of materials including, but not limited to plastics, metals, woods, composite materials or rubber in various physical forms to reduce the size of the material to that suitable for further handling or processing. Material to be ground is usually placed inside a hopper which drops or feeds material into a space bounded by a ram at a first end and a rotor at a second end. The ram is then moved forward to force the material toward a driven rotor having a plurality of cutters (or teeth) thereon.
Furthermore, a grooved screen, or a plurality thereof, may be employed at an outlet proximate the rotor wherein each screen has a plurality of V-shaped grooves or like apertures therein through which the ground material passes once it is reduced in size such that it is capable of passing through the screen. Often several screens are provided for a given grinder, and the screen having the appropriate groove size is selected for use depending upon the material being ground. The screens are typically positioned proximate a discharge area below the rotor to allow ground material to exit the grinding process by dropping into a bin or like container for further processing.
The cutters travel in the screen grooves, thereby permitting the cutters to remove excess material from the screen so that ground material, for example, plastic, does not block the apertures therein. Material that is not reduced in size sufficiently to pass through the screen is simply re-ground until it passes through the grooves and into a collection bin or the like.
Rotary grinders that utilize driven rams to force feed material towards a rotor are known in the art. A compartment proximate the lower portion of the hopper of the grinder contains a pair of horizontal channels or grooves on opposed sides of the hopper. A horizontal ram, conventionally driven by a hydraulic cylinder and piston system, is forced forward to move the material to be ground toward the rotating rotor with its plurality of cutters.
A grinder operator initially monitors the progress of a given grinding operation to determine how quickly to move the ram forward in order to efficiently grind material and establishes parameters for the grinder operation, usually programmed (or selected) into a control computer via an operator interface. The ram moves forward (usually stopping and starting) and attempts to control the operation based upon a preselected motor amperage of the rotor during grinding. When the rotor becomes loaded down with too much material, the rotor motor can trip off due to blown fuses or thermal overloads. Furthermore, in many instances the material being ground may begin to melt due to the heat buildup involved. This problem is particularly acute when grinding various types of plastics, materials with low melting points, materials that tend to wrap themselves around the rotor, or materials with high coefficients of friction such as rubber. Often the result is partially melted plastic clogging the cutters and screen grooves, thereby drastically diminishing the cutting action of the rotor.
Even when such clogging doesn't occur, if the material is fed towards the rotor too quickly the rotor motor must work too hard to grind the material thus decreasing the working life of the cutters and the motor. In contradistinction, if the material is fed too slowly, the grinding operation uses more energy than necessary to efficiently grind the material or doesn't grind material at all, thereby reducing productivity and enhancing the time and expense of the grinding operation.
German patent reference no. 3,932,345 teaches a typical rotor and cutters. U.S. Pat. No. 4,844,363 teaches a non-horizontal moving hopper ram wherein the ram is guided by a pair of grooved tracks 49. U.S. Pat. Nos. 5,509,613 and 5,344,088 teach material grinders having a ram driven by a pair of pistons 24 from hydraulic cylinders 23. U.S. Pat. No. 5,645,234 teaches another grinder having a ram head.